Pressurized water nuclear reactors comprise, inside a reactor building, a vessel enclosing the core of the reactor filled with pressurized water and a primary circuit constituted by a plurality of loops in communication with the core. Each of the loops of the primary circuit comprises a steam generator in which the pressurized water is cooled by heating and vaporizing feed water. Steam generators are disposed in shielded compartments arranged inside the reactor building.
The steam generators of each of the loops are connected to the vessel by inlet and outlet pipes for pressurized water under termed primary pipes. These pipes are connected to the water box in the lower part of the steam generator by elbows and nozzles constructed upon the forming of the water box of the steam generator. These connecting elbows are welded by one of their ends to the end of the primary pipe and by their second end to the nozzle for connection to the water box of the steam generator.
After a certain period of operation, the steam generators, which enclose a bundle of thermal exchange tubes ensuring the separation between the primary water and the feed water, must be inspected and, if some of the tubes of the bundle present leak, these tubes must be closed by a plug or sleeved to avoid contamination of the feed water.
When the steam generator has been in operation for a long period, it may be necessary to replace some or all of the tubes of the bundle. Since plugging of an excessive number of tubes of the bundle of the steam generator may result in an unacceptable reduction in the thermal performances of the steam generator and sleeving of the tubes can only be carried out in certain zones inside or in the vicinity of the tube plate of the steam generator.
A tube replacement operation is complex and it has appeared preferable, in certain cases, to completely replace the steam generators of pressurized water nuclear reactors. This replacement operation requires cutting off the primary pipes in the vicinity of the connection nozzles of the worn steam generator and welding the nozzles of the new replacement steam generator to the primary pipes awaiting reconnection.
FR-A-2,614,462 discloses a process for completely replacing a steam generator of a pressurized water nuclear reactor which permits, by means of topometric measurements carried out on the new replacement generators and on the elements fixing and connecting the worn steam generators inside the shielded compartments, optimizing the operations for the cutting of the primary pipes and the connection of the steam generators.
By using the process disclosed in FR-A-2,614,462, it was possible to carry out the replacement operations on a worn steam generator in nuclear power stations under perfectly optimized conditions as concerns the duration of the time required to carry out all of the operations necessary for changing a steam generator, the quality of these operations and limitation of the irradiation doses received by the personnel employed for the replacement operations.
In particular, such a replacement operation on the steam generators of a pressurized water nuclear reactor was carried out in a perfectly satisfactory workshop manner in the first half of 1990 at the Dampierre nuclear power station, at Bugy in 1993 and at Gravelines in 1994.
However, the steam generator replacement operations are generally carried out by connecting the nozzles of the new steam generator to the ends of the elbows placed at the ends of the primary pipes awaiting reconnection, so that the elbows of the primary circuit are generally not replaced at the same time as the steam generator.
Certain-check ups and inspections have shown that the elbows connecting the water box of the steam generator to the primary circuit undergo in service deteriorations which require their replacement. Of course, owing to the complexity of these replacement operations, which require cutting the pipes of the primary circuit, it is much easier to replace the elbows at the same time as the steam generator.
To replace an elbow at the same time as a steam generator, it is necessary to cut the primary pipe at each of the ends of the elbow and to fix the replacement elbow in a position interposed between the end of the primary pipe awaiting reconnection and a nozzle of the steam generator. This replacement operation for an elbow, generally designated "closer", is very complex; in particular, the handling of the elbow up to the butt joint position requires the development of tooling complicated to use when the steam generator is in position. The welding operation is all the more delicate as it must be carried out inside the shielded compartment of the steam generator, at each of the ends of the elbow and at the same time as the welding of the second primary pipe of the steam generator.
The operation is even more complex when it is necessary to replace both elbows connecting the steam generator to the primary circuit.
While it was possible to replace a steam generator by making only two cuts in the primary circuit, the replacement of a steam generator at the same time as the replacement of one or both elbows requires making three or four cuts, and the same number of connections by welding.